Emission semiconductors

The use of MIP films with emissive semiconductor substrates can potentially impart selectivity to PL and EL responses. To illustrate, in addition to the PL enhancements caused by ammonia and trimethylamine (Figs. 2a and 2b), other amines like methylamine and dimethylamine elicit a similar PL response when adsorbed onto the bare surface of etched n-CdSe. These amines cannot generally be reliably distinguished with this technique because of the similarity of their PL... 

By inserting a semiconductor x-ray detector into the analysis chamber, one can measure particle induced x-rays. The cross section for particle induced x-ray emission (PIXE) is much greater than that for Rutherford backscattering and PIXE is a fast and convenient method for measuring the identity of atomic species within... 

The light emitted in the spontaneous recombination process can leave tire semiconductor, be absorbed or cause additional transitions by stimulating electrons in tire CB to make a transition to tire VB. In tliis stimulated recombination process anotlier photon is emitted. The rate of stimulated emission is governed by a detailed balance between absorjDtion, and spontaneous and stimulated emission rates. Stimulated emission occurs when tire probability of a photon causing a transition of an electron from tire CB to VB witli tire emission of anotlier photon is greater tlian that for tire upward transition of an electron from tire VB to tire CB upon absorjDtion of tire photon. These rates are commonly described in tenns of Einstein s H and 5 coefficients [8, 43]. For semiconductors, tliere is a simple condition describing tire carrier density necessary for stimulated emission, or lasing. This carrier density is known as... 

The uncertainty principle, according to which either the position of a confined microscopic particle or its momentum, but not both, can be precisely measured, requires an increase in the carrier energy. In quantum wells having abmpt barriers (square wells) the carrier energy increases in inverse proportion to its effective mass (the mass of a carrier in a semiconductor is not the same as that of the free carrier) and the square of the well width. The confined carriers are allowed only a few discrete energy levels (confined states), each described by a quantum number, as is illustrated in Eigure 5. Stimulated emission is allowed to occur only as transitions between the confined electron and hole states described by the same quantum number. 

MaxweU-Boltzmann distribution. At high electric fields, E > 10 kV/cm, no longer increases with electric field and approaches a limiting saturation speed, determined primarily by optical phonon emission. Eigure 3 shows the variation of drift speed with electric field for electrons and holes in various semiconductors. 

A light-emitting diode (LED) is a forward-biasedp—n junction in which the appHed bias enables the recombination of electrons and holes at the junction, resulting in the emission of photons. This type of light emission resulting from the injection of charged carriers is referred to as electroluminescence. A direct band gap semiconductor is optimal for efficient light emission and thus the majority of the compound semiconductors are potential candidates for efficient LEDs. 

Laser sources that emit in the mid-ir region of the spectmm (2—5 -lm) are useful for detection of trace gases because many molecules have strong absorption bands in that region. Other appHcations include remote sensing and laser radar. Semiconductor lead—salt (IV—VI) lasers that operate CW at a temperature of 200 K and emission wavelength of 4 p.m are commercially available however, they have relatively low output powers (<1 mW) (120). 

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